1. Field of the Invention
This invention relates generally to “back-end” processing of GMR read heads, including the processing of assemblies containing GMR read heads, and particularly to a method of preventing damage to the transverse biasing of the GMR read heads by process induced stresses and by external magnetic fields that may be present during such processing.
2. Description of the Related Art
GMR (giant magneto-resistive) and MR (magneto-resistive) heads and slider assemblies are formed on wafers in arrays of complete, already magnetized units, which must then be subjected to additional, so-called “back-end” process steps, such as separation of the array into individual units and lapping each unit to an acceptable degree of smoothness. An example of how at least a substantial portion of such “back-end” processing proceeds is provided by Sasaki et at. (U.S. Pat. No. 6,374,479), who teach a method of slicing the wafer into rows of slider sections and of bonding the sliced sections to a supporting plate for further processing.
The heads, themselves, are small and delicate and subject to various types of damage during the back-end processing steps. The industry has been particularly concerned with damage to the heads caused by electrostatic discharges (ESD) that can occur during the processing. In this regard, Girard et al. (U.S. Pat. No. 6,146,813) teach a method of forming (and removing) shunts between portions of electrical components (including GMR heads), said shunts providing a mechanism for safely dissipating induced currents and electrostatically deposited charges. Han et al. (U.S. Pat. No. 6,415,500) teaches a method of avoiding ESD by connecting the sensor to its shields during the back-end processing steps in such a way that there is an equal electrical potential between the sensor and its shields during the duration of the process.
Another concern during back-end processing is that the lapping process can proceed beyond the desired limits and damage the active surfaces of the GMR head. Zhu (U.S. Pat. No. 6,230,389) teaches the formation of a lapping monitor, which is an additional, sacrificial portion of the sensor layer whose changing shape during lapping allows the progress of the lapping to be accurately followed.
None of the methods discussed above address the problem of possible adverse affects of back-end processing to the magnetic properties of the GMR head. Even before the beginning of back-end processing, the magnetic properties of GMR layers have been established by annealing in the presence of appropriate magnetic fields. Magnetic biasing is of particular importance to the performance of a GMR head and two types of biasing are established prior to back-end processing: longitudinal and transverse. Longitudinal biasing, typically provided by adjacent permanent magnetic layers formed with the conducting lead layers, stabilizes the domain structure and magnetic moment direction of a GMR head's free layer. Transverse biasing, typically provided by a antiferromagnetically pinned layer formed within the GMR sensor, provides a reference direction with respect to which the magnetic moment of the free layer moves. Both of these biasing structures are already established during the wafer formation prior to back-end processing.
In back-end processing, the lapping process produces stresses and even plastic deformations within the wafer as an unavoidable part of stock removal. These stresses cause both biases to change, particularly when combined with other disturbances such as stray magnetic fields, elevated temperatures and ESD induced currents. As a result, the final GMR product can have its biases altered in a random and uncontrollable fashion, adversely affecting product yields. While the longitudinal bias can be restored to its pre-processing state with relative ease, restoration of the transverse bias cannot be accomplished economically either during or after the processing. Prior art methodology, as noted above, has concentrated on prevention of ESD events during processing. The present invention teaches an entirely new method for controlling damage to biasing during back-end processing and, by so doing, will improve product yields substantially.